That temp could be real. With no SLI all the work is being done by the one card. If it's the card with the fan sandwiched between the two boards it is going to be choked for fresh (cool) air. Since the first slot is the one with the fan crowded and most likely the default card for non-SLI it sounds reasonable.
We run stuff with die temps up to 105 in aerospace but we don't expect them to last very long.
Open the case and blow a fan on the 2 video cards and see if the 83 comes down much. If it doesn't I'd swap the cards between slots once a month to extend the life of the hotter running card.
It's a bug with the newer beta drivers,and unproven drivers. No one knows for sure if it's a real temp or not. My 6800 does the same thing,which is why I won't use beta drivers anymore,and because there are too many bugs to bother with.